vb.net barcode generator Collins, Daniel. Carrier Grade Voice Over IP. New York: McGraw-Hill, 2001. p. 95 96. in Software

Encoder QR Code ISO/IEC18004 in Software Collins, Daniel. Carrier Grade Voice Over IP. New York: McGraw-Hill, 2001. p. 95 96.

Collins, Daniel. Carrier Grade Voice Over IP. New York: McGraw-Hill, 2001. p. 95 96.
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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Table 2-1 Descriptions of voice codecs (ITU)
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ITU Standard P.800 G.114 G.165 G.168 G.711 G.722 G.723.1 G.729 G.729A H.323 P.861 Q.931
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Description A subjective rating system to determine the Mean Opinion Score (MOS) or the quality of telephone connections A maximum one-way delay end to end for a VoIP call (150 ms) Echo cancellers Digital network echo cancellers PCM of voice frequencies 7 kHz audio coding within 64 Kbps A dual-rate speech coder for multimedia communications transmitting at 5.3 and 6.3 Kbps Coding for speech at 8 Kbps using conjugate-structure algebraic codeexcited linear-prediction (CS-ACELP) Annex A reduced complexity 8 Kbps CS-ACELP speech codec A packet-based multimedia communications system Specifies a model to map actual audio signals to their representations inside the human head Digital subscriber signaling system number 1 ISDN user-network interface layer 3 specification for basic call control
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Table 2-2 Parameters of voice codecs
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Standard G.711 G.721, G.723, G.726 G.728 G.729 G.723.1
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Data rate (Kbps) 64 16,24,32,40 16 8 5.3, 6.3
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Delay (ms) 0.125 0.125 2.5 10 30
MOS 4.8 4.2 4.2 4.2 3.5, 3.98
Codec Waveform
Popular Speech Codecs Codecs are best known for the sophisticated compression algorithms they introduce into a conversation. Bandwidth costs service providers money. The challenge for many service providers is to squeeze as much traffic as possible into one pipe, that is one channel. Most codecs allow multiple conversations to be carried on one 64 kbps channel. There is an inevitable trade off in compression for voice quality in the
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conversation. The challenge for service providers is to balance the economics of compression with savings in bandwidth costs.
G.711 G.711 is the best-known coding technique in use today. It is a wave-
form codec and is the coding technique used in circuit-switched telephone networks all over the world. G.711 has a sampling rate of 8,000 Hz. If uniform quantization were to be used, the signal levels commonly found in speech would be such that at least 12 bits per sample would be needed, giving it a bit rate of 96 Kbps. Nonuniform quantization is used with eight bits used to represent each sample. This quantization leads to the well-known 64 Kbps DS0 rate. G.711 is often referred to as PCM. G.711 has two variants: A-law and mu-law. Mu-law is used in North America and Japan where T-Carrier systems prevail. A-law is used everywhere else in the world. The difference between the two is the way nonuniform quantization is performed. Both are symmetrical at approximately zero. Both A-law and mulaw offer good voice quality with a MOS of 4.3, with 5 being the best and 1 being the worst. Despite being the predominant codec in the industry, G.711 suffers one significant drawback; it consumes 64 Kbps in bandwidth. Carriers seek to deliver voice quality using little bandwidth, thus saving on operating costs.
G.728 LD-CELP Code-Excited Linear Predictor (LD-CELP) codecs implement a filter and contain a codebook of acoustic vectors. Each vector contains a set of elements in which the elements represent various characteristics of the excitation signal. CELP coders transmit to the receiving end a set of information determining filter coefficients, gain, and a pointer to the chosen excitation vector. The receiving end contains the same code book and filter capabilities so that it reconstructs the original signal. G.728 is a backward-adaptive coder as it uses previous speech samples to determine the applicable filter coefficients. G.728 operates on five samples at one time. That is, 5 samples at 8,000 Hz are needed to determine a codebook vector and filter coefficients based upon previous and current samples. Given a coder operating on five samples at a time, a delay of less than 1 millisecond is the result. Low delay equals better voice quality. The G.728 codebook contains 1,024 vectors, which requires a 10-bit index value for transmission. It also uses 5 samples at a time taken at a rate of 8,000 per second. For each of those 5 samples, G.728 results in a transmitted bit rate of 16 Kbps. Hence, G.728 has a transmitted bit rate of 16 Kbps. Another advantage here is that this coder introduces a delay of 0.625 milliseconds with an MOS of 3.9. The difference from G.711 s MOS of 4.3 is imperceptible to the human ear. The bandwidth savings between G.728 s 16 Kbps per conversation and G.711 s 64 Kbps per conversation make G.728 very attractive to carriers given the savings in bandwidth.
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The Public Switched Telephone Network (PSTN)
The Public Switched Telephone Network (PSTN)
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